1. Field of the Invention
This invention relates to a process for the deposition of thin films. More particularly, this invention relates to a process for deposition of such films using supercritical fluids.
2. Prior Art
There are several developed techniques which are used for thin film deposition. The most important are chemical vapor deposition and the vacuum deposition. However, several problems are associated with these methods which compelled researchers to investigate new routes for thin film preparation.
One of the most serious problems associated with chemical vapor deposition and vacuum deposition is that these methods result in the deposition of atoms or very simple molecules only. Moreover, chemical vapor deposition requires exotic starting materials and both chemical vapor deposition and vacuum deposition require high temperatures which are disadvantageous. An additional disadvantage of vacuum deposition is the requirement of sophisticated equipment for a high vacuum operation, and a disadvantage of both of these prior art methods is the necessity to use supports of specific geometrical shapes. Another disadvantage of chemical vapor deposition is contamination of films by heterogeneous elements present in a vapor phase.
Historically, interest in supercritical fluids was related to the observation that such fluids were often excellent solvents in the same manner as normal liquids. As a result most of the proposed industrial applications were associated with the extraction of the specific products from liquid and solid mixtures. At present more than 100 processess which employ this idea are patented. Decaffeination of coffee, extraction of light oils from residual oils and coal, certain classess of chemicals from natural products, organics from water and oligomers from polymers are the most often mentioned examples of supercritical fluid applications.
In addition to the above, the unusual properties of super critical fluids stimulate attention of investigators in the "non-traditional" areas. A process concept to utilize the pressure-dependent solvation power of supercritical fluids to comminute materials was reported in 1981 Chem. Eng. News, vol. 59, (31), pp 16-17 (1981). In the industry, comminution of materials is carried out by grinding or by precipitation from solution. However, many chemicals are sensitive to these processes because of temperature effects or because of co-precipitation of impurities from liquid stream. Supercritical fluids nucleation offers the potential to tailor particle size and size distribution without temperature and solvent impurity limitations. Attractive candidates for comminution by super critical fluid nucleation are heat labile dyes, fine chemicals, pharmaceuticals and intermediates which must be formed in some specific particle size for subsequent processing or use.
German patent No. 2,853,066.7 describes a process for covering the surface of porous powders or porous bodies and fabrics with protective or decorative layers by contacting the material with a gas in the supercritical state as a liquid medium. The gas contains the solid or liquid covering material in solution.
Quite a different application for supercritical fluids is in the hydrothermal breeding of synthetic quartz crytals in supercritical water at about 670.degree. K. and 100-200 MPa (Williams D. F., Chem. Eng. Science Vol 36, 11, p. 1769 (1981)). In a wider context it has been forecast that supercritical extraction will find application in the upgrading refractories, particularly when used in combination with liquid solvents.